Process for recovering sulphur dioxide from waste gases



June 6, 1939. H. F. JoHNs'roNE ET AL 2,161,056

PROCESS FOR RECOVERING SZJLPHR DIOXIDE FROM WASTE GASES Filed March 24,1957 Come QQ u 542 55 ma@ eugj Coface fave/iron- Patented June 6, 1939`Unirse `srrirrss PATENT corri/acs ,PRooEss FoaRilcova-mnsA sULPHUnDroxmn FROM W-As'r-E GASES" Henry F. Johnstone and Alamjit D, Singh, Ur-

bana, Ill., assignors, by mesne assignments, toY Commonwealth EdisonCompany, Chicago, Ill.`, a corporation of'lllinois i Applieatimn March24, 1937, serial No. 132,692

claims; (c1. 23e-17s) This' invention relates'to the removal of sulcostof using the processes is-` far greater than phur dioxide from Wastegases, such as boiler -the value of the by-product recovered. furnacegases, smelter gases,pancl those from the In` the copending' applicationof` Henry F. chemical process industries,.. although` its'use isJohnstone, SerialNoL 665,337, led AprilflO; 1933,

plicable for use in the removal and vrecovery'oi" prising theabsorption, at alow'temperature'with sulphur dioxide from any gasescontaining this respect to the process, of sulphur dioxide and'anvconstituent. i i aqueous solution containing the salt' of anlia'ci'dlrI'his gas has become one of the most obnoxious having ionizingcharacteristics such that its and undesirable constituents oi wasteYgases.v It ionization either decreases less than'that of sul- 10constitutes an annoyance on account of its phurous acid with increasingtemperature or, pollution of the atmosphere in communities adpreferably,its ionization decreases With: increase jacent industrial centers. Thecommercial use of in temperature, is disclosed. In that'appli'catio'n,sulphur has threatened exhaustion of the Worlds there is disclosed theprocess'in which ammonium l5 sulphur supply and, inasmuch as the largestpart sulphite is added to the washing solution, vvhere-Y 15 of theindustrial use of sulphur passes thro-ugh by`upon the absorption ofsulphur dioxide, theV the oxidation stage represented by the dioxide,solution contains sulphite` and bisulphite ions. the removal andrecovery of the dioxide from The solution is thereafter heated andtherewaste gases and the like in industrial localities action isreversed, liberating the absorbedsulis economically advantageous. phurdioxide. However, there'are certain' re- 20 In' the practice of thepresent invention, We quirements in the use ofthe process as disclosedhave been able to remove and recover pure sulin said copendingapplication which the present phur dioxide gas from Waste gases having,by application is intended to overcomeand-to provolume, as loW as 0.05%*of this constituent duce an improved method ofremoving andrepresent. Thenew process is especially useful, covering the sulphur dioxidev from theWaste 25 therefore, in recovering sulphur dioxide from gases. In theprocess of recovering sulphur gases vhaving' a low percentage of thisconstituent dioxide from waste gases by the absorption'of 'the present,such as gases resulting from the comgas in'a solution containing asoluble sulphite bustion of coal, oil, orv other sulphur bearing andbisulphite 'and the releasing of the gas by phur dioxide, although thegas may also `be rethe Vsmall extent'to which the dissolvedsulpl'iur`moved and recovered from smelter fumes and the dioxide can be removedfrom the' solution `Withlike. out excessively increasing the heatArequired for In considering the problem of removing sulthe regeneration.In th'e case of' ammonium notlimited to such gases, since itis equally,apnow PatentV 2,082,006 an improved process' coin- 5A of). fuels, andwaste industrial'gases containing sulheating the solution, thelimitationY is' denitely 30V.

- phur4 dioxide Afrom gases, several-general systems sulphiteebisulphitesolutions, the stripping" ofthe 35A have been employed. One'ofthesecomprises the sulphur dioxide from the solution' cannot becarwashing of the gases containing sulphur dioxide ried beyond a certainratio of sulphiteto bisul= with Water, or with Water containing analkaline phite even by using large quantities of heat, or' re-agent orother solvent, to recover the sulphur Steam, Owing t0 the volatility ofammonia-'over 40 dioxide. This system hasb'een improvedv upon,solutionsV of ammonium sulphite at high tein- 40,

as disclosed in the Johnstone patent, No. peratures. It is apparent alsothat'regen'eration 2,021,936, issued November 26,- 1935, which is byheating must'alway's depend on' vaporization directed to the oxidationof the sulphur dioxide ofl the'sulphur diXde and', therefore,'some'small in the water to sulphuric acid, which then mayV equilibriumvapor pressure ofv sulphur dioxide' 451 be removed as desired. Anothermethod commust remain Vover the regenerated solution'. 45'

prises the chemical reduction of the sulphur When this solutionis cooledbeforelreturning' to dioxide to free sulphur, which has not, however,the scrubber, Y the vapor pressure of sulphur'y proved practical. Asanother expedient, various dioxide is lowered', but it'st'illremainsni'te. The mechanical means have beenV attempted in order Wastegases'leaving the scrubber 'in vContact with' 505 to remove and recoverthe sulphur dioxide from theA entering solution consequently cannot beVsuch gases, such as absorption on a porous mate` completely denuded ofthe sulphur dioxide. Re'- rial, or by compression of the gases, orrefrigerageneration by heating', therefore, canno'tbel usedv tion. Atthe present time, however, none of these for gases containing `only afew partsper million processes appears to` have solved theI problem' ofsulphur dioxide.

; adequately, since in large scale 'operation' the It is an object of'the present inventionr to pro- 55'? Vide a solvent for absorbing thesulphur dioxide which will readily absorb the gas Without producing aninsoluble precipitate in the scrubber. In connection with this step ofthe process, a second object comprises the removing of the dissolvedsulphur dioxide from a solution of a sulphite and a bisulphite, therebyincreasing the alkalinity of the solution.

The substitution of a chemical process of regeneration of thesulphite-bisulphite solution for the heating process disclosed in thecopending Johnstone application would remove these disadvantagesinherent in the latter process and would, in turn, have the followingadvantages:

1. Faster rate of absorption of the sulphur dioxide frorn the wastegases due to the greater absorption gradient between the gases and theabsorbent.'

2. More eicient removal of the sulphur dioxide from the gases.

3. The possibility of use of the process on more dilute gases.

4. Obviation of the necessity for saturating the solution with sulphurdioxide as nearly as possible in order to regenerate economically.

5. The maintenance of strict proportionality between the cost ofregeneration and the amount of sulphur dioxide recovered.

6. The possibility of advantageous use of sodium sulphite-bisulphitesolutions over the ammonium solutions, thus reducing the cost of wastagelosses.

In carrying out the process disclosed in the present application, wecontemplate the use of a metallic oxide for the chemical regeneration ofthe sodium sulphite-bisulphite absorbing solution, and the recovery ofthis oxide and the absorbed sulphur dioxide, whereby the absorbingsolution can be regenerated without the disadvantages attendant upon theregeneration and liberation in former processes.

Various objects and advantages of the improved process of the presentinvention over the previous processes will appear more fully from thefollowing detailed description, which, taken in conjunction with theaccompanying drawing, will disclose to those skilled in the art theparticular operation and results attendant upon use of a preferredembodiment of our invention.

The single figure of the drawing illustrates the specic application ofthe process to the removal and recovery of sulphur dioxide from boilerflue gases, and is intended to represent diagrammatically a flow sheetfor the present process.

Considering the ow sheet in detail, there is provided a scrubber orwashing tower i, provided with a suitable gas inlet 2 which may beconnected to receive stack gases from boilers or the like, and which isprovided with a gas outlet 3 through which the gases are exhausted fromthe scrubber. A suitable series of nozzles or other discharge devicesfor the absorbing solution is disposed adjacent the upper end of thescrubber, as indicated at 4, and at the lower end of the scrubber thereis provided a hopper or sump 5 in which the solution, together with theimpurities and solids washed out of the gas stream, collects.

In employing the process, we first scrub the waste gases in the scrubberl with an aqueous solution of sodium sulphite and bisulphite introducedinto the upper end of the scrubber and discharged in countercurrentdirection with respect to the flow of gases entering the gas inlet 2 andbeing exhausted through the outlet 3. Preferably the scrubber isprovided With a suitable checkerwork or packing, as may be desired, inorder to provide intimate contact between the absorbing solution and thegases. After the solution is collected in the sump 5, the collectedsolution, with the suspended solids therein, is removed through the line6 into a settling cone l, in which cone the suspended solids areprecipitated from the solution, and are withdrawn in any suitablemanner. The absorbed sulphur dioxide of the solution, which exists inthe solution as an increased ratio of bisulphite to sulphite, is removedfrom the settling cone l through the line 8 and the force pump 9 to asuitable proportioner l2 connected to the pump 9 through the conduit l0.

From the proportioner l2, the solution containing the absorbed sulphurdioxide is discharged through the conduit I5 into a suitable mixer I6which contains rotating paddles or agitating members diagrammaticallyindicated at il, where the solution is intimately mixed with a solidmetallic oxide capable of forming an insoluble or slightly solublesulphite. This oxide is added to the solution from a storage containeri8 through a suitable line i9, and agitation is provided to promote therapid dissolution of the oxide in the mixer i6. The solution is thenconducted from the lower end of the mixer I6 through the line 2i) intothe upper end of an agitator 22 having agitating elements driven fromthe shaft 23, where formation and growth of the sulphite crystals arepromoted, and the mixture is discharged through the line 2Q to asettling device 25, in which the greater portion of the insolublesulphite is removed from the lower end thereof through the line 25 to afilter member 28.

The clear supernatant liquor, now having the original ratio of sulphiteto bisulphite, is withdrawn from the settling cone 25 through the line2l to the pump 30, and from this pump is returned through the conduit 32to the discharge nozzles :i in the scrubber l for further absorption ofsulphur dioxide.

It will be apparent that this part of the process, therefore, operatesin a cyclic manner, there being no consumption of the absorbing agentother than possible evaporation of water in the scrubber due to the heatof the waste gases, which can be made up or supplied at any point in thecycle by the addition of make-up water. Incidental losses of solution byspray or leak may also be made up by addition of the alkaline absorbingagent, in the preferred embodiment of the invention, this being sodiumsulphite and bisulphite. For example, either sodium carbonate, or sodiumhydroxide may be added to the solution. Either of these is immediatelyconverted to sodium sulphite by the sulphur dioxide absorbed from thegases.

Returning now to the precipitated metallic sulphite settling in the cone25, this may either be withdrawn from the process and discarded, if onlythe removal of sulphur dioxide from the Waste gases is considered, ormay be saved as a valuable product, or if desired, and preferably, thisprecipitate is treated to recover the original metallic oxide in anothercyclic procedure.

If the non-cyclic process is chosen, lime may be used as the oxide, inwhich case calcium sulphite is the insoluble material precipitated fromthe scrubbing solution. Because of the diniculty of recovering lime fromthe precipitated sulphite, however, zinc or magnesium oxide arepreferred if the cyclic procedure for recovery of the metallic oxide istobe employed.

Starting with the preeipitateds-lurry'or sludge in the settling cone215,-y the cyclicV procedure-con-y sists of filtering and washingthe/solids on acon-4 tinuous iilter device-28 of'- known `form, thefiltrate from this device being returned through the line 29 to the pump3D and thencebeing mixed with the clear supernatant absorbingsolutionfrom the cone 25,` the two absorbingsolutions being united atthe pump 30 and returned through the line 32 to the scrubber I. Thesolids from the filter 28' are dried in fa drier 3'4, c'onnec't'e'dtbthe filter 28 through the line 33,*and after being dried in. the unitv34, theV driedlsolid.. isl transmitted through the line 36 toacalcinerindicated generally at 3l. In this calciner the solid is subn jectedtoindirect heat within a drum 39 b-y a gas fed burnerd or the like, Athevcalcination producing thev metal-lic'roxides and'liberated sulphurdioxide'.VV VThe detailsA of: this phase of the proc-ess areVdisclosedvv in4 the copending` application of narnia-D. Singh,serial-No. 148,074, nieu June 14', 1937. Y

VThe vliberated sulphur dioxide is conducted through the line 43 tosuitable coolers anddriers through the line 44, and may be thensubjected to compression or the like in order to lid-uefy the same Vforproducing they valuable ley-product of the process. The oxide recoveredin the calciner 3i is returned through theline 38 to the storagecompartment I8, fromY whichit is again metered out through the line I0into the mixer i6 in contact with the saturated absorbent solution fromthe proportioner l2. The use of sodium sulphite solutions for absorbingsulphur dioxi'de'is generally known, as is alsothe use of limesuspension for removing the absorbed sulphur dioxide as a precipitate.However, the use'of a metallic oxide-for regenerating the sodiumsulphi-te solution inthe manner described is believed novel, especiallywhen the oxide itself which is used for the regenerating phase of theabsorption solution can also be subjected to a cyclic process whereby-itis regenerated forfurtheruse; Y v Considering the process in connectionwith the useoi lime iorregenerationof the sodium sulphite solution, aparticularly important `novel feature resides in the obvia-tion of thenecessity of `,circulating suspensions of solids through the scrubber,which is a difficult operation because ofthe tendency of the solids toAadhere to the scrubbing surfaces and clog the scrubber. Yet the netresult of the process'is thesame as if the lime were sent directly tothe scrubber, that is, thesulphur dioxide is removed from the gases byindirect action with lime.

Thusit will be seen that the present invention contemplates the removalof sulphur dioxide from flue gases by the use of an absorbing solutionfree from any suspended or partially precipitated solids which wouldtend to destroy the efficiency of the scrubbing operation, and yet limeis employed for removing the absorbed sulphur dioxe ide from theabsorbing solution. rIhis is distinctly different from the directtreatment of the flue gases with a lime suspension in the scrubber,which soon results in inefficient scrubbing action due to the cloggingVaction of the solids.v

When lime is used for regenerating the ab.- sorbent, the material is ofcourse consumed, since it is impracticable to attempt to recover thesulphur dioxide from the lime Suspension. Since theregenerating mediuml'is' consumed; a quantity quivalent torthe sulphur dioxidefremoved fromthe waste gases must be supplied?. For theproduction of puresulphurdioxide by the process; it

is desirable, therefore, to use in placeA o'fthe lime a metallic oxideof which the dry sulphite is unstable upon heating. Of these, zincrandmagnesium sulphites are examples which` may be employed. Because of thelower decomposition temperature of the sulphite andthe smaller tendencyto form side products, we nd that theuse of zinc oxide as a regeneratingmedium is preferred. i

As an example of the action of Zincoxideon a solution containing sodiumsulphite and'bisul-I phite, one of the tests which we conducted gave thefollowing results: Y .i

The original solution coming from the scrubber contained 2.97 mols ofsodium ions per 100 mols of water, and the molar solution ratio ofsulphur dioxide to sodium was 0.915. The solution, therefore, was mostlyin the bisulphite form. By the addition of Zinc oxide in a molar ratioof 0.35 to dissolved sulphur dioxide, the ratio oi sulph`ur dioxide tosodium in the solution was reduced to 0.615, and the ratio of sulphurdioxide removed to zinc oxide added was 1.0. The decrease inthe acidityof the solution represented by th-e increase in the pH was from 5.06 to6.41. The solution, therefore, was restored to the more alkalinecondition with the higher ratio of sulphite to bisulphite and wassuitable for scrubbing gases containing sulphur dioxide.

In another example, representing a continuous operation of the scrubbingcycle, gases containing approximately 0.18% sulphur dioxide werescrubbed with a solution of sodium sulphite and bisulphite containing1.25 mols of sodium ions per 100 mols of water. The gases were flowingthrough the scrubber at the rateof 300 cubic feet per minute and thesolution at the rate ofr 12.2 pounds per minute. lution werecountercurrent. The ratio of dissolved sulphur dioxide to sodium in thesolution` entering the scrubber was maintained at 0.60 by the additionof Zinc oxide in amounts equivalent to that of the sulphur dioxideabsorbed. The efficiency of sulphur dioxide absorption under suchconditions varied between 95 and 99%.

We have found that the composition and concentration of the scrubbingi,solution may be varied at will and chosen to best fit the particular gasand type of scrubber employed. In general, the best results appear to beobtained when the ratio of sulphur dioxide to sodiumv in theregenerating solution entering the scrubber is not allowed to fall below0.6. On the other. hand, in order to keep the scrubber operating at itshighest efliciency, the same ratio in the solution leaving the scrubbermust not be allowed to rise above 0.9. By kno-wing the quantity ofsolution required for wetting the scrubber surfaces and the quantity andcomposition of gas to be scrubbed, the concentration of the solution maybe readily determined. v

While the flow sheet illustrates the use of zinc oxide in theregeneration of the absorbing solution, as we have stated lime ormagnesium oxide may be employed for application to the solutions ofsodium sulphite and bisulphite. We recognize, however, that any othermetallic oxide of a class capable of forming an insoluble or slightlysoluble sulphite that decomposes with heating may be employed in theprocess. Examples of this class are the oxides of barium, strontium,lead, and cadium. Furthermore, the scrubbing solution may contain thesulphite and bisulphite The flow of gas and so of other ions thansodium, such as lithium, potassium, and ammonium. n

In the operation of the process as thus far described, it may happenthat a small portion of the sulphite present in the scrubbing solutionis converted to the sulphate by o-xidation. Sulphate may also accumulatein the solution from other sources, such as by absorption of sulphuricacid from the waste gases simultaneously with the absorption of thesulphur dioxide. Since in some cases, such as when zinc oxide is used,the sulphite is preferentially removed from the solution by theprecipitating agent, the net effect is an accumulation of sodiumsulphate in the solution with a resulting decrease in the sodiumconcentration available for forming the sulphite and bisulphite. It istherefore desirable that some process of removing the sulphate beemployed, in order to increase the sodium concentration or to restorethe same in order to provide a proper quantity of sulphite andbisulphite ions.

We have found that this removal can be accomplished easily bywithdrawing through the line i3 a portion oi the scrubbing solutionleaving the settling cone 'E'. This portion of the scrubbing solution,which has been partially saturated with the sulphur dioxide absorbed inthe scrubber I from the waste gases, can be completely saturated with aportion of the sulphur dioxide coming from the calciner 3? through theline d3 and the line 45. A suitable saturator It is provided, thesulphur dioxide from the line i5 being sprayed through the dischargemeans 46 within the saturator It into the solution entering through theline i3, to produce an acidity of the' absorbing solution such thatcalcium sulphite will not be precipitated. From the saturator Hl, theacidiiied solution is transmitted through the line t? into an agitatorSii, containing the rotating agitating means t9. In this agitator, theacidied solution is contacted with a concentrated solution of calciumbisulphite through the line 5, which may be introduced from. anysuitable source. This produces a precipitation of calcium sulphate,which is removed from the solution by passing the mixture of calcium.bisulphite and acidified absorbing solution through the line 52 into asettling cone The precipitate of calcium sulphate is removed in the cone53 and is transmitted through the line M to the filter 55, where it iswashed and drained away through the line il. The clear liquor ordesulphated solution is returned from the settling cone 53 through theline 56, and from the filter 55 through the line 5'! to a return pump 5Ewhich is connected through the line 59 with the conduit i@ leading fromthe scrubber to the proportioner i2.

While the process of sulphate removal asI a side phase of the processhas been described in detail, it is to be understood that otherbisulphites may be employed for this purpose. For example, bariumbisulphite may be substituted for calcium bisulphite in this phase ofthe process and will accomplish removal of the accumulated sulphates inthe absorbing solution, in order to maintain the sodium concentration atthe proper point to provide suicient sulphite and bisulphite ions forproper interaction with the waste gases.

This process of removng the sulphate from the sulphite-bisulphitesolution employed in absorbing sulphur dioxide from the waste gases hasbeen described generally in the copending Johnstone application, SerialNo. 665,337 now patent 2,082,- 006. However, in the instant case, thecalcium or barium bisulphite solution is added to the saturated solutionrather than the adding of 'lime or calcium carbonate and subsequentsaturation of the solution by sulphur dioxide.

In the process as thus disclosed, the following chemical reactions takeplace in the various portions of the cycle:

In the scrubber:

NazSOs-l-SOz-i-HzOZNaI-ISOs In the sulphate removal:

In the regenerator:

In the calciner:

ZnSO3 ZnO+ SO2 It is believed apparent that, by the present process, wehave provided an efficient and economical manner of removing andrecovering sulphur dioxide from waste gases, such as stack gases,smelter gases and waste gases from the chemical process industries. Bythe use of the present process, the absorbing solution is maintained ata substantially constant concentration, rendering the efliciency ofabsorption substantially constant throughout the process. Further, itwill be noted that the process involves two subprocesses, each of whichis cyclic in operation. Thus, the absorbing solution, after beingsaturated with the SO2 from the waste gases, is regenerated by theaddition of a metallic oxide thereto, which precipitates sulphitecrystals and restores the absorbing solution for return in a regeneratedcondition to the scrubber for further contact with the waste gases. Themetallic oxide itself is precipitated as a sulphite, and is thensubjected to drying and heating to decompose the same into the metallicoxide and the free sulphur dioxide, which may be removed and collectedin any suitable manner, such as by being cooled and compressed toliquefy the same. The regenerated metallic oxide is returned to asuitable storage chamber for recontact with the absorbing solution toregenerate this solution. As a side step in the process, we employ theuse of calcium or bariiun bisulphite for removing accumulated sulphatesin the absorbing solution, in order to maintain the sodium concentrationat the proper point to provide sufficient sulphite and bisulphite ionsfor proper interaction with the waste gases to absorb the sulphurdioxide.

We are aware that various modifications and changes may be made in theprocess described and illustrated herein, without departing from theessential features of the present invention, and We therefore do notintend to be limited except as disclosed and defined in the appendedclaims.

We claim:

l. 'Ihe process of recovering sulphur dioxide from dilute sulphurdioxide gases which consists of absorbing the sulphur dioxide in anaqueous solution of sodium sulphite and bisulphite, treating theenriched solution with zinc oxide, removing the precipitated zincsulphite from the solution, returning the solution to contact with thegases in a cyclic manner, drying the zinc sulphite, heating the zincsulphite to decomposition into zinc oxide and sulphur dioxide,collecting the sulphur dioxide, and returning the zinc oxide for furthertreatment of the enriched solution.

2. The process of removing sulphur dioxide from an aqueous solutioncontaining sodium sulphite and sodium bisulphite which consists ofadding zinc oxide to precipitate rzinc sulphite, removing theprecipitated zinc sulphite, drying the zinc sulphite, heating the drysulphite to decomposition into the zinc oxide and sulphur dioxide, andreturning the zinc oin'de for treatment of the said sulphite solution,in a cyclic manner.

3. The process of removing sulphur dioxide from dilute sulphur dioxidegases, which comprises: absorbing the sulphur dioxide in an aqueoussolution containing a sulphite and a bisulphite, treating the enrichedabsorbing solution with zinc oxide to form Zinc sulphite, removing thezinc sulphite from the sulphite and bisulphite solution, returning thesolution for recontact with the gases, drying the zinc sulphite, heatingthe zinc sulphite to decompose the same into zinc oxide and sulphurdioxide, and returning the zinc oxide for treatment of the absorbingsolution in a cyclic manner.

4. The continuous process of removing and recovering sulphur dioxidefrom dilute sulphur dioxide gases, which comprises: absorbing thesulphur dioxide in an aqueous solution containingsodium sulphite, sodiumbisulphite, and Vsulphate, separating a portion of the enrichedabsorbing solution, saturating said portion with sulphur dioxide gas,treating the saturated solution with calcium bisulphite solution,separating the precipitated calcium sulphate from the treated portion ofsaid solution, combining said portion with the main solution, treatingthe entire solution with zinc oxide, removing the precipitated zincsulphite from the solution, returning the solution for recontact Withthe gases, drying the zinc sulphite, heating the zinc sulphite todecomposition into Zinc oxide and sulphur dioxide, collecting thesulphur dioxide, and returning the zinc oxide for further treatment ofthe entire solution.

5. 'I'he continuous process of removing sulphate and sulphur dioxidefrom an aqueous solution containing a sulphite, a bisulphite and asulphate, which comprises: contacting a portion of said solution withsulphur dioxide to saturate the same, treating said saturated portion ofthe solution With calcium bisulphite, separating the precipitatedcalcium sulphate from said portion of said solution, returning thebalance of said portion of said solution to the main solution, adding tothe entire solution zinc oxide to precipitate zinc sulphite, removingthe precipitated zinc sulphite, drying the zinc sulphite, heating thedried zinc sulphite to decomposition into zinc oxide and sulphurdioxide, and returning said Zinc oxide for treatment of said entiresolution.

HENRY F. JOI-INSTONE. ALAMJIT D. SINGI-I.

